systemctl,loginctl,machinectl: add --signal=list

This lists numerical signal values:
$ systemctl --signal list
SIGNAL NAME
1      SIGHUP
2      SIGINT
3      SIGQUIT
...
62     SIGRTMIN+28
63     SIGRTMIN+29
64     SIGRTMIN+30

This is useful when trying to kill e.g. systemd with a specific signal number
using kill. kill doesn't accept our fancy signal names like RTMIN+4, so one
would have to calculate that value somehow. Doing
  systemctl --signal list | grep -F RTMIN+4
is a nice way of doing that.

src/shared/parse-argument.c

@@ -1,7 +1,9 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include "format-table.h"
 #include "parse-argument.h"
 #include "signal-util.h"
+#include "stdio-util.h"
 #include "string-table.h"
 #include "string-util.h"
 
@@ -16,6 +18,29 @@ int parse_signal_argument(const char *s, int *ret) {
                 return 0;
         }
 
+        if (streq(s, "list")) {
+                _cleanup_(table_unrefp) Table *table = NULL;
+
+                table = table_new("signal", "name");
+                if (!table)
+                        return log_oom();
+
+                for (int i = 1; i < _NSIG; i++) {
+                        r = table_add_many(
+                                        table,
+                                        TABLE_INT, i,
+                                        TABLE_SIGNAL, i);
+                        if (r < 0)
+                                return table_log_add_error(r);
+                }
+
+                r = table_print(table, NULL);
+                if (r < 0)
+                        return table_log_print_error(r);
+
+                return 0;
+        }
+
         r = signal_from_string(s);
         if (r < 0)
                 return log_error_errno(r, "Failed to parse signal string \"%s\".", s);